1. Field of the Invention
The present invention relates to developers for pressure-sensitive recording and to developing sheets containing the same. In greater detail, the present invention relates to novel developers for pressure-sensitive recording which comprise a polyvalent metal salt of salicyclic acid substituted by a 1,1-diarylalkyl group or a triarylmethyl group and developing sheets containing the same.
2. Description of the Prior Art
A pressure-sensitive recording system in practical use at the present time comprises microcapsules containing in solution a colorless or slightly colored electron donating dye precursor (hereinafter, a "color former") and an electron attracting material (hereinafter, a "developer") as described for example in U.S. Pat. Nos. 2,712,507, 2,730,456 and 2,730,457. Formation of developed images is accomplished by contacting the color former contained in a dissolved state in the microcapsules with the developer upon destruction or rupture of the capsules.
Hitherto, though clay minerals such as acid clay, attapulgite or bentonite, etc., phenols and phenol-formaldehyde resins have been used as developers, they all suffer several serious defects. For example, the developing ability of clay minerals deteriorates with time and the light fastness and water resistance of the developed images are poor. The developing ability of the phenols is low and their stability with time is poor. Further, phenol-formaldehyde novolak resins have low developing ability for couplers such as leuco Auramine type couplers or spiropyran type couplers, etc., and they produce images which are easily yellowed by light and the light fastness of the developed images is poor.
Recently, it has been reported in Japanese Patent Publications Nos. 10,856/74 and 25,174/76 and U.S. Pat. No. 3,934,070 that polyvalent metal salts of salicylic acid derivatives are excellent developers. Of these developers, however, the metal salts of salicylic acids having low oil-solubility and low molecular weight do not exhibit good stability to hydrolysis (i.e., there is a reduction in their developing ability resulting from hydrolysis of the metal salts to form free acids), they can be developed by water (e.g., color formation takes place when wetted as the color former contacts the developer surface) and they suffer a deterioration in developing ability with time, etc. Further, with salicylic acid derivatives having 12 or more carbon atoms having a large oil-solubilizing group (described in Japanese Patent Publication No. 25,174/76), though the above described defects are somewhat overcome, the developing ability and definition of the developed images are not adequate, and thus developers having higher developing ability have been required. For example, zinc 4-pentadecylsalicylate and zinc 5-octadecylsalicylate have insufficient developing ability.